1. Field of the Invention
The present invention relates to a ranging apparatus and a ranging method, and more particularly to a ranging apparatus and a ranging method for detecting the phase delay of reflected light from an object that is irradiated with modulated light at each of the pixels of an image capturing device, for thereby detecting a three-dimensional structure of the object.
2. Description of the Related Art
One known process for measuring the distance up to an object is an optical TOF (Time Of Flight) ranging process.
As shown in FIG. 18 of the accompanying drawings, a ranging apparatus based on the optical TOF ranging process comprises a light source 200 in the form of an LED array, for example, for emitting intensity-modulated light (modulated light), an image capturing device 204 for detecting reflected light from an object 202 irradiated with the modulated light from the light source 200, and an optical system 206 for focusing the reflected light onto the image capturing device 204.
If the modulated light emitted from the light source 200 and applied to the object 202 is intensity-modulated, for example, at a high frequency of 20 MHz, then the modulated light has a wavelength of 15 m. When the modulated light travels back and forth over a distance of 7.5 m, the modulated light as it is detected by the image capturing device 204 has undergone a phase delay of one cycle length.
The phase delay that the reflected light undergoes with respect to the modulated light will be described below with reference to FIG. 19 of the accompanying drawings.
As shown in FIG. 19, a reflected light R has a phase delay of φ with respect to a modulated light W. In order to detect the phase delay of φ, the reflected light R is sampled at four equal intervals, for example, in one cyclic period of the modulated light W. If the sampled amplitudes of the reflected light R at respective phases of 0°, 90°, 180°, 270°, for example, of the modulated light W are represented by A0, A1, A2, A3, respectively, then the phase delay of φ is expressed by the following equation:φ=arctan {(A3−A1)/(A0−A2)}
The reflected light from the object 202 is focused onto the light-detecting surface of the image capturing device 204 by the optical system 206. The light-detecting surface of the image capturing device 204 comprises a two-dimensional matrix of pixels (photodiodes). When the phase delay of φ is determined at each of the pixels according to the above equation, a three-dimensional structure of the object 202 can be detected.
A ranging apparatus based on the above principle is disclosed in Japanese Patent No. 3758618, for example.
The disclosed ranging apparatus measures the distance from the apparatus to an object when reflected light from the object is detected in exposure periods established in a plurality of patterns by opening and closing the overflow drain gates (OFDG) or readout gates of an image capturing device out of phase with each other.
Specific details of the ranging process performed by the disclosed ranging apparatus will be described below with reference to FIGS. 20 and 21A through 21D of the accompanying drawings. In a first frame, in response to a negative-going edge of a synchronizing signal Sa (see FIG. 21A) in step S1 shown in FIG. 20, the light source 200 emits a modulated light W in step S2. When the object 202 is irradiated with the modulated light W, the object 202 reflects it as a reflected light R to the image capturing device 204. As shown in FIG. 21A, the image capturing device 204 is adjusted to have the center of a first exposure period Tr synchronized with a time point that is a time period T1 later than the negative-going edge of the synchronizing signal Sa, i.e., a time point at which the modulated light W has a phase of 0°. The image capturing device 204 is also adjusted such that each of successive exposure periods Tr thereof has a cycle length of 2π.
In the first frame, the amount of reflected light R at the time the phase of the modulated light W is 0° is photoelectrically converted into an electric charge, which is stored in the image capturing device 204 in step S3. In a next second frame, the electric charge stored in the image capturing device 204 is transferred as an analog signal, and the analog signal is converted into a digital signal in step S4. The digital signal is saved in a buffer memory as a sampled amplitude A0 of the reflected light R at the time the phase of the modulated light W is 0° in step S5. Then, the light source 200 stops emitting the modulated light W in step S6.
Thereafter, in a next third frame, in response to a negative-going edge of the synchronizing signal Sa (see FIG. 21B) in step S7, the light source 200 emits the modulated light W again in step S8. When the object 202 is irradiated with the modulated light W, the object 202 reflects it as a reflected light R to the image capturing device 204. As shown in FIG. 21B, the image capturing device 204 is adjusted to have the center of a first exposure period Tr synchronized with a time point that is a time period T2 (>T1) later than the negative-going edge of the synchronizing signal Sa, i.e., a time point at which the modulated light W has a phase of 90°. The image capturing device 204 is also adjusted such that each of successive exposure periods Tr thereof has a cycle length of 2π.
In the third frame, the amount of reflected light R at the time the phase of the modulated light W is 90° is photoelectrically converted into an electric charge, which is stored into the image capturing device 204 in step S9. In a next fourth frame, the electric charge stored in the image capturing device 204 is transferred as an analog signal, and the analog signal is converted into a digital signal in step S10. The digital signal is saved in a buffer memory as a sampled amplitude A1 of the reflected light R at the time the phase of the modulated light W is 90° in step S11. Then, the light source 200 stops emitting the modulated light W in step S12.
Thereafter, in a next fifth frame, in response to a negative-going edge of the synchronizing signal Sa (see FIG. 21C) in step S13, the light source 200 emits the modulated light W again in step S14. When the object 202 is irradiated with the modulated light W, the object 202 reflects it as a reflected light R to the image capturing device 204. As shown in FIG. 21C, the image capturing device 204 is adjusted to have the center of a first exposure period Tr synchronized with a time point that is a time period T3 (>T2) later than the negative-going edge of the synchronizing signal Sa, i.e., a time point at which the modulated light W has a phase of 180°. The image capturing device 204 is also adjusted such that each of successive exposure periods Tr thereof has a cycle length of 2π.
In the fifth frame, the amount of reflected light R at the time the phase of the modulated light W is 180° is photoelectrically converted into an electric charge, which is stored into the image capturing device 204 in step S15. In a next fourth frame, the electric charge stored in the image capturing device 204 is transferred as an analog signal, and the analog signal is converted into a digital signal in step S16. The digital signal is saved in a buffer memory as a sampled amplitude A2 of the reflected light R at the time the phase of the modulated light W is 180° in step S17. Then, the light source 200 stops emitting the modulated light W in step S18.
Thereafter, in a next seventh frame, in response to a negative-going edge of the synchronizing signal Sa (see FIG. 21D) in step S19, the light source 200 emits the modulated light W again in step S20. When the object 202 is irradiated with the modulated light W, the object 202 reflects it as a reflected light R to the image capturing device 204. As shown in FIG. 21D, the image capturing device 204 is adjusted to have the center of a first exposure period Tr synchronized with a time point that is a time period T4 (>T3) later than the negative-going edge of the synchronizing signal Sa, i.e., a time point at which the modulated light W has a phase of 270°. The image capturing device 204 is also adjusted such that each of successive exposure periods Tr thereof has a cycle length of 2π.
In the seventh frame, the amount of reflected light R at the time the phase of the modulated light W is 270° is photoelectrically converted into an electric charge, which is stored into the image capturing device 204 in step S21. In a next eighth frame, the electric charge stored in the image capturing device 204 is transferred as an analog signal, and the analog signal is converted into a digital signal in step S22. The digital signal is saved in a buffer memory as a sampled amplitude A3 of the reflected light R at the time the phase of the modulated light W is 270° in step S23. Then, the light source 200 stops emitting the modulated light W in step S24.
The phase delay φ of the reflected light R is determined based on the sampled amplitudes A0, A1, A2, A3 stored in the buffer memory, and the distance from the ranging apparatus to the object 202 is determined based on the phase delay φ in step S25.
If the above ranging apparatus is incorporated in a digital camera, a surveillance camera, or the like, then the ranging apparatus is exposed to noise such as clock noise, etc. from a display circuit, other IC circuits, etc.
Since the frames have different exposure timings as shown in FIG. 22 of the accompanying drawings, noise spikes 210 may be added to the exposure periods Tr of a certain frame and noise spikes 210 may not be added to the exposure periods Tr of another frame. Consequently, it is difficult to remove those noise spikes 210. In the example shown in FIG. 22, the noise spikes 210 are added to the exposure periods Tr of the third and fifth frames.
In addition, inasmuch as it is necessary for the different frames to have the different time periods T1 through T4 from the negative-going edge of the synchronizing signal Sa to the center of the first exposure period Tr, a complex drive circuit is required to operate the image capturing device 204, and the ranging process is burdensome for the CPU used therein.